This invention relates to the production of olefins by the thermal cracking of heavy hydrocarbon mixtures wherein the starting mixture is first subjected to hydrogenation.
To produce olefins, it is conventional and advantageous to employ light hydrocarbons, such as, for example, ethane or propane, or hydrocarbon mixtures having a boiling point of below 200.degree. C., such as, for example, naphtha, as starting materials for a thermal cracking operation. These starting materials result in a high yield in olefins and relatively few undesirable by-products.
However, in view of the high demand for olefins, which may lead to a short supply and increase in price of the aforementioned advantageous starting materials, several attempts have been made through the years to develop processes which permit the utilization of higher-boiling starting materials.
When employing such higher-boiling charges, the olefin yield is reduced and the yield of liquid hydrocarbons boiling above 200.degree. C. is increased. The proportion of the latter high-boiling fraction, which is difficult to treat in further operation, increases significantly with the boiling point of the starting material. In addition, further difficulties are encountered in that higher-boiling starting materials lead to increased formation of coke and tar. These products are deposited on the walls of the conduit elements, for example, pipelines and heat exchangers, thereby impairing heat transfer, and furthermore resulting in constrictions in cross section. It is therefore necessary to conduct a removal of these deposits more frequently than when using light hydrocarbons.
In order to solve this problem, DOS [German Unexamined Laid-Open Application] No. 2,164,951 describes a process wherein the starting material is catalytically hydrogenated prior to the thermal cracking thereof. By virtue of this pretreatment, there is affected a reduction in the content of aromatic compounds in the starting material, otherwise leading to undesired cracked products. Moreover, a desulfuration of the starting material occurs.
In U.S. Pat. No. 3,898,299, a process is described wherein atmospheric petroleum residue feedstock is hydrogenated, then subjected to vacuum distillation to recover a distillate boiling below 650.degree. C. at atmospheric pressure, and only this distillate is subjected to thermal cracking. In this patent, it is also pointed out on Column 1, lines 45-47 that the carbon in the vacuum residue is lost to olefins production.